Regarding wireless power transmission technologies, various methods have been proposed according to a frequency band of an electromagnetic wave that is a transmission medium, and a transmission efficiency or a transmission distance is different according to a transmission method and a transmission apparatus used therein.
In a case where a frequency band of an electromagnetic wave is as low as several MHz at most, an electromagnetic induction type wireless power transmission is mainly used. In this method, a pair of coils is disposed adjacent to each other, and a magnetic field line generated at one coil passes through the other coil. In order to increase the transmission efficiency, it is necessary to make a pair of coils as close as possible and reduce an axis shift as much as possible, such that a magnetic field line passes through the other coil as many as possible. This method has already been put to practical use because the transmission efficiency is high and the miniaturization of the apparatus can be achieved.
In a case where a frequency band of an electromagnetic wave is about 100 KHz to several hundreds of MHz, an electromagnetic field resonance method is suitable. In NPL 1, two spiral antenna resonators, of which both ends are opened, are disposed opposite to each other, and perform power transmission mostly by magnetic coupling. Herein, experiments have been conducted by using three types of spiral antennas, of which resonance frequencies are 122 KHz, 13.56 MHz, and 1.49 GHz, and relationships between a transmission distance and a transmission efficiency at each resonance frequency have been compared. In the experiment of 122-KHz resonance frequency, perhaps because a Q value of a resonator constituting a coil is low, the transmission efficiency is 90% or less even when the transmission distance is 100 mm. The transmission efficiency is reduced up to 50% when the transmission distance is 1,000 mm. Furthermore, when the transmission distance is increased, the transmission efficiency is rapidly reduced. In the experiment of 13.56-MHz resonance frequency, the transmission efficiency of 90% or more is maintained when the transmission distance is up to 220 mm. Furthermore, when the transmission distance is increased, the transmission efficiency is rapidly reduced. The transmission distance at which the transmission efficiency is 50% is about 300 mm. Also, in the experiment of 1.49-GHz resonance frequency, perhaps because a Q value of a resonator constituting a coil is low or a radiation occurs, the transmission efficiency is 90% or less. The transmission efficiency is rapidly reduced when the transmission distance is 3 mm or more. The transmission distance at which the transmission efficiency is 50% is about 4 mm. It can be seen from these experiments that in the resonance-based power transmission using the coil, the transmission efficiency is highest at the frequency band around ten to twenty MHz. Also, since this method is relatively tolerant to the shift of two coil resonators in an axial direction, the accuracy as high as the electromagnetic induction method is not required in the arrangement of the coil resonators.
The two methods as described above transmit power to a region corresponding to a distance before a generated electromagnetic field, called a near field (short-distance field), is still formed as an electromagnetic wave that is a radio wave. Also, a far field (long-distance field) corresponding to a region farther than the near field is a region in which an electromagnetic wave is radiated as a radio wave. The near field corresponds to a region in which a distance is about λ(2π)=λ×0.16 or less, where λ is a wavelength of an electromagnetic wave. In the experiments described in NPL 1, the transmission distance at which the transmission efficiency is around 50% is λ× 1/2460 when the resonance frequency is 122 KHz, λ×0.014 when the resonance frequency is 13.56 MHz, and λ×0.02 when the resonance frequency is 1.49 GHz, and power is all transmitted within the range of the near field.
As described in NPL 2, in the wireless power transmission experiment by Massachusetts Institute of Technology (MIT), which was announced in 2007, power to turn on a 60-W bulb was transmitted at the transmission distance of 2 m. However, the resonance frequency at that time was 9.90 MHz. Therefore, the transmission distance is only λ×0.066, and this is also power transmission within the near field.
Also, PTL 1 proposes a method that transmits energy by using a spherical dielectric resonator. In this method, energy transmission is performed by configuring and arranging two spherical dielectric resonators in the region of the near field such that one is coupled under a condition of a radiation caustic surface of the other.
On the other hand, as the method that transmits power to the region of the far field by using the radio wave, a method using a microwave of several hundreds of MHz or more and a rectenna antenna has been developed. As represented by a space solar power satellite (SPS), a radio wave generated by converting high power obtained by solar power generation in space into a microwave is formed in a beam shape by using an antenna array, is transmitted to the earth from a position located 3,600 Km away, and is received by a rectenna antenna. A system for carrying out this method requires a plurality of antennas and needs to control a phase of power supplied to the respective antennas. This is large-scale and expensive. Research has been conducted to apply this technology to a system for charging a battery of a parked electric vehicle, but this is also expensive as in the SPS.
Also, PTL 2 discloses a resonator device in which a dielectric resonator is disposed within a cutoff waveguide such that an axis of the dielectric resonator is matched with a propagation direction of electromagnetic energy in the cutoff waveguide.
On the other hand, in order to improve the efficiency of power transmission between two resonators, it is preferable that electromagnetic energy generated from one resonator is received to the other resonator while leaking out as little as possible. However, since an electromagnetic field generated from one resonator tends to diverge in, in particular, an outer peripheral portion, an electromagnetic field passing through the other resonator is reduced as much. Therefore, when attempting to increase the distance between the two resonators, in particular, so as to increase the transmission distance, the influence of the electromagnetic field divergence is increased as described above, which tends to be an obstacle to the improvement in the power transmission efficiency. As described in PTL 3 and PTL 4, a radio wave lens or a dielectric lens, which is made of a dielectric material, is generally disposed so as to suppress the divergence of an electromagnetic wave.